The invention relates to a heater, for example: a wood burning stove, with relatively thin walls and interior surfaces that are exposed to combustion gases coated with a heat resistant anti-oxidant coating.
Heating appliances using various hydrocarbon fuels have been used for heating air in dwellings, heating water, creating steam and many other functions. While the present invention is described herein using the example of a thin metal walled wood burning stove for heating the surrounding ambient air, it will be understood that the invention is equally applicable to many types of heating devices that burn oil, gas, pellets, corn, propane, biogases, or sawdust to heat air or liquids in domestic, commercial or industrial applications as well.
For the most part, the primary function of wood burning stoves until recently was for cooking and heating. While this is still true today, other factors have driven the design of stoves to produce heat more efficiently while discharging less of the undesirable byproducts of combustion into the atmosphere. Prior art stoves have traditionally had the capability of producing large amounts of heat, whilst simultaneously producing large undesirable amounts of noxious substances which were expelled into the atmosphere by the burning fuel.
In order to provide an acceptable life for most of the prior art wood burning stoves, manufacturers usually provided a metallic shell in which firebricks and/or heavy iron castings were formed and fitted into a firebox in order to protect and shield the inner surface of the outer metallic shell of the stove from the burning fuel. The resultant stove tended to be quite massive, slow to heat and difficult to move. Because of the massiveness of these stoves, considerable heat energy is required just to raise the temperature of the stove to the desirable operating temperature.
Recently stove manufacturers resorted to producing an xe2x80x9cairtightxe2x80x9d stove which limited the amount of combustion air allowed to the firebox so that a firebox filled with wood could be made to burn at a controlled rate for many hours.
Because of the lack of oxygen supplied to the burning wood, these xe2x80x9cairtightxe2x80x9d stoves tended to produce copious amounts of creosote and other gaseous products resulting from incomplete combustion of the burning fuel because of oxygen starvation. The low temperature of the emitted flue gas also allowed creosote and other noxious substances to be deposited in the cold chimney flue.
Continued use of these xe2x80x9cairtightxe2x80x9d stoves usually resulted in a chimney fire from time to time. Because of the problems associated with this type of heating appliance, environmental authorities had little choice but to introduce stringent restrictions on the types of stoves which could be sold in each jurisdiction.
In 1988 the U.S. Environmental Protection Agency introduced a set of standards for New Residential Wood Heaters under Title 40 Code of Federal 15 Regulations Part 60, which has had a great influence on the design of stoves which have been and are to be introduced into the U.S. market. The presence of these Regulations has provided stove manufacturers all over the world with a set of guidelines to measure the efficiency of any wood burning stove and the resulting production of any undesirable emitted materials produced by the stove under test during a monitored burning operation so as to enable a comparison of the test stove results against a set of given standards.
It is with a view to the production of a stove which is able to easily meet the 40 C.F.R. (60) regulations that this invention is directed.
In U.S. Pat. No. 4,941,451, a stove having a firebox which is surrounded by multiple air chambers is described. Primary air enters the front of the stove just below the door and is ducted to the top of the firebox where it is directed downwardly from a point well above the burning fire to induce combustion of the fuel in the firebox.
Cooling air for the stove also enters the stove in an opening in the bottom of the stove below the firebox floor. A fan is shown propelling air entering the opening into three separate streams.
A first stream is ducted up the back of the stove behind the firebox and across the top of the stove and out to the room via louvres.
A second stream is ducted upwardly in a pair of riser tubes to empty from a manifold above the fire but below the hollow baffle. Air leaves a secondary manifold to ignite and burn unburned gases.
A third stream enters the hollow baffle from a side space. This air cools the baffle and exits through a series of holes above the second secondary stream.
A slider type draft control adjusts the amount of primary air fed to the firebox. The secondary air is pressurized by a fan in the plenum beneath the firebox floor.
U.S. Pat. No. 4,832,000 uses separate primary and secondary airflows to improve the combustion of the fuel in the firebox. Both primary and secondary airflows are preheated.
In U.S. Pat. No. 4,665,889, a stove having a baffle and separate primary and secondary airflow paths is illustrated. The primary air is not really heated, but the secondary air is heated during its passage through the secondary duct work.
The objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
A heater for burning fuel having a hollow enclosure with an air inlet, and a combustion gas outlet. The enclosure has an exterior heat exchange wall exposed to ambient air and an interior surface defining a combustion chamber. The enclosure has a body with front and rear openings into which are fitted a front panel and a rear panel defining joints therebetween. The interior surface of the enclosure is coated and joints are sealed with heat resistant anti-oxidant material, for example: porcelain enamel, having a co-efficient of thermal expansion not substantially greater than the co-efficient of thermal expansion of the enclosure. The enamel used should be a pyrolytic enamel as opposed to architectural enamels which cannot withstand the high temperatures of the firebox.
This invention is directed to a stove which is extremely lightweight (in comparison to the heavy stoves of recent vintage) and typically uses sheet steel as the basic material for forming an enclosure for a typical stove fire box. The interior of the sheet material forming the firebox is preferably coated with a layer of a pre-selected material which is resistant to break down due to exposure to high temperature and the products of combustion present in a firebox. The sheet steel which forms the firebox of the stove of this invention is typically coated with a protective layer of a suitable heat resistant anti-oxidant coating material on the inside surface to protect the steel sheet from the effects of exposure to the high temperatures existing in a firebox and the combustion byproducts produced therein. The sheet steel is typically a mild steel with low carbon content which lends itself to the heat resistant anti-oxidant coating process which must be carried out in an oven at temperatures approaching 1540xc2x0 F. The heat resistant anti-oxidant coating may selected to be a high temperature pyrolytic porcelain enamel or glass which contains a small amount of titanium (up to about 8%) which tends to have the effect of making the interior heat resistant anti-oxidant coating surface of the firebox self cleaning. The heat resistant anti-oxidant coating and the metallic sheet steel base material must have complementary co-efficient of expansion that are selected in order that the heat resistant anti-oxidant coating steadfastly adheres to the base material during the many temperature excursions to which the heat resistant anti-oxidant coated sheet steel will be subjected over the life of the stove. A variation in the choice of metal for the substrate will usually dictate a corresponding variation in the coating to ensure that the coating is not subjected to excessive residual tension or residual compression as the metal substrate and the coating cool after firing or plasma arc deposition is completed.
The stove is provided with primary and secondary inlet air passages which are designed specifically to control the quantities of primary and secondary invitiated air allowed to enter the combustion chamber of the stove during a normal combustion process. The secondary inlet air is ducted through passages in the stove which are placed so as to be in excellent heat transfer relationship with the burning fuel in the combustion chamber of the stove so as to efficiently heat the air in the duct work to a temperature approaching or matching that existing on the combustion chamber of the stove.
The primary air (unheated) enters the stove above the access door and is ducted downwardly so as to sweep downwardly against the inside surface of the glass on the access door. This tends to prevent any buildup of smoke particles on the glass in the door. Because of the difference in density of the cold inlet air and the hot air near the burning fuel, the inlet air tends to make its way to the bottom of the firebox to promote primary combustion.
The stove of this invention is provided with a forwardly extending baffle which extends from the rear of the combustion chamber and which is fastened into the combustion chamber at each side of the baffle to the interior of the stove at some distance beneath the exhaust vent. This baffle prevents the hot air produced during the burning process from exiting directly from the fire into the exhaust vent and up the flue. Because the hot gases produced by pyrolysis must linger longer in the hot combustion chamber, the chances for ignition of these gases to occur is much greater in the presence of the baffle.
The secondary air enters the stove through a draft control (at the front of the stove) and passes through a heat exchanger duct or preheat heat exchanger to the rear of the stove which allows the secondary air to undergo a preheating operation during its passage to the rear of the stove. This preheated air next enters a heat exchanger (at the back of the stove) where the air passing through the heat exchanger is heated to a temperature approaching the maximum temperature in the rear combustion chamber wall. This heated air is allowed to exit from the heat exchanger from preferably two sets of exit ports.
Some of the heated secondary air exits the heat exchanger of the stove from exit ports formed in the heat exchanger just below the point of intersection of the baffle. The balance of the secondary air may be ducted forwardly in the stove toward the front of the combustion chamber in a duct associated with the baffle and which is provided with suitable exit ports in the baffle so that heated air is expelled from these exit ports near the front of the stove.
It is the combination of the admittance of these predetermined volumes of primary and secondary air in the presence of the baffle which determines the efficiency and the U.S.E.P.A. rating of the stove during a burning operation.
Prior art stoves, resulting from their higher insulating value during monitored burning operation, have significantly higher flue gas temperatures, resulting in lower heating efficiencies than that produced by the present invention while simultaneously meeting safety and low emissions criteria.
In a first embodiment of this invention, there is provided a wood-burning stove having a combustion chamber which is the general shape of a barrel resting on its side. The interior surfaces of the stove which are exposed to the hot exhaust gases are coated with a suitable heat resistant anti-oxidant coating such as high temperature resistant pyrolytic porcelain enamel or glass material. The rear and bottom walls are specially designed closure members for high efficiency and low emissions. The front closure member has an opening formed therein for providing access to the combustion chamber. The rear closure member is formed into a heat exchanger. The above structure is supported on a base which is incorporated into the structure, and which is provided with a set of legs. The combustion chamber is provided with a baffle to control the flow of the hot gases before exit through the exhaust vent.